Abstract/Summary

This report describes the results of a geophysical study undertaken in order to characterise mine tailings at the Frongoch Mine near Devil’s Bridge, Ceredigion, Wales. This is part of ongoing research into the environmental impact of abandoned metalliferous mines in the Central Wales Orefield in the UK.
The site ranks highly in a list of abandoned mine sites, which were recognised by the Environment Agency for England and Wales as posing the greatest threat to surface waters (Environment Agency, 2002). Electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) surveys were carried out at Frongoch Mine in February 2008. A detailed understanding of the lateral extent of the historic tailings lagoon, the thickness of the tailings and any potential impact of the mine waste on the underlying bedrock and the surrounding environment was obtained. The resistivity data clearly show the highly conductive signature of the superficial tailings deposits and good correlation is observed with the outline of the lagoon recorded on historic maps. The depth extent of conductive material measured at the centre of the lagoon is of 12-13 m bgl and is found to exceed the suspected base of the lagoon. Intrusive investigation is required in order to corroborate the geophysical information. There is evidence to suggest that significant fine structure resolved in some of the shallow deposits corresponds to observations made in trial pits located in the vicinity of the geophysical profiles.
The wider context of this work relates to the increasingly stringent regulation of mine waste under EU environmental legislation, in particular the Water Framework Directive and the Mine Waste Directive. Both address the environmental impacts of abandoned mine sites, including mine spoils and tailings, which often represent a significant diffuse source of pollution. Geophysical techniques can provide helpful insight through non-invasive mapping of the extent and volume of mine tailings and any contamination associated with them.